1. Field of the Invention
This invention relates to improvements in the production of thermosetting resinous polyepoxides, including processes and products resulting therefrom.
2. Summary of the Prior Art
Since their discovery, thermosetting resinous polyepoxides, i.e., epoxy resins, have in industries and scientific disciplines found application in many forms, principally as surface-coating materials combining toughness, flexibility, adhesion and chemical resistance to a nearly unparalleled degree.
The epoxy resins are fundamentally polyethers, but retain their epoxy nomenclature on the basis of their starting material and the presence of epoxide groups in the polymer before crosslinking or curing. The most common types of resinous polyepoxides are produced by reaction of monomeric epoxy compounds, chiefly epichlorohydrin, with dihydric phenols, chiefly bisphenol-A, to give diglycidyl ethers.
Depending upon molecular weight, the resinous polyepoxide may vary from a viscous liquid to a high melting solid. The higher molecular weight resinous polyepoxides can be made by a process known as "upgrading" or "advancement". In such an upgrading or advancement process, an initial liquid resinous polyepoxide is reacted with a dihydric phenol in the presence of a catalyst until enough of the dihydric phenol is incorporated into the epoxy polymer chain to increase molecular weight to the desired level.
Such upgrading processes have in the past been conducted both on a batch basis and on a continuous basis. See, for example, U.S. Pat. Nos. 3,547,881 and 3,919,169. In such known batch and continuous upgrading processes, the dihydric phenols and liquid polyepoxide together with a catalyst are admixed or otherwise contacted at a relatively low temperature and then heated up to the reaction temperature and held at reaction temperature for a time sufficient to produce the resinous polyepoxide of higher molecular weight.
In such known batch and continuous upgrading processes, however, cycle times are typically relatively lengthy. For example, batch reactions involving bisphenol-A and a liquid polyepoxide consisting essentially of the diglycidyl ether of bisphenol-A can take from about 10 to about 20 hours for the reaction to be completed. The continuous process described in U.S. Pat. No. 3,919,169 involves a shorter time on the order of about 2 hours, but in a continuous process it would be highly advantageous if the reaction time could be shortened significantly below this level.
In addition to economies of time, such long cycle or reaction times can lead to a relatively wide molecular weight distribution which may in turn lead to end use disadvantages. For example, surface coating imperfections or "orange peel" has been observed when molecular weight distribution and concomitant viscosity characteristics are not properly controlled.